Panel boards for floors are made as cross-veneered boards in order to prevent excessive distortions due to moisture and heat influences after being laid.
In this connection the known panel boards have, besides the tread ply that forms the actual tread surface, a crossband ply glued therewith, which is formed from wooden boards or slats and the fiber grain of which extends in each case at right angles to that of the tread ply.
In order to prevent the so-called "dishing" of such panel boards, the crossband ply is cut through in the longitudinal direction of its fiber grain by incisions running at right angles to the tread surface and intended to prevent the situation that working of the crossband ply at right angles to its fiber direction leads to arching of the panel board.
In the manufacture of such panel boards, a sheet is first manufactured, for example by means of a heatable press, from tread and crossband ply, into which sheet mutually parallel slots are sawed from the surface facing away from the tread ply, which slots must on the one hand completely pass through the crossband ply but on the other hand are not to penetrate into the tread ply if at all possible.
This machining requires a high degree of accuracy and is therefore expensive.
This machining is effected by a multi-blade circular saw, in which a plurality of circular saw blades are mounted side-by-side on a rotating machine shaft.
Since the working width of such a multi-blade circular saw is limited, the sheet machined herewith also can have only this limited width.
The sheet machined in this way is then sawed into individual panel boards, which if necessary are additionally subjected to finish-machining, for example surface grinding of the tread surface, milling of grooves and tongues into the side edges and the like.
The saw cuts are made in the fiber direction of the tread ply, i.e., at right angles to the incisions, so that the length of the panel boards manufactured in this way most agrees with the working width of the multi-blade circular saw.
However, there is also another reason why the manufacture of longer panel boards is not possible:
During gluing, the individual wood constituents are exposed to the moisture of the glue and possibly also to heat, so that the sheet manufactured by gluing can warp before the glue has fully hardened. As a result of the moisture of the glue, subsequent warping usually occurs even after hardening of the glue.
In practice, the warping can still be evened out only by finish-machining by a material-removing process; thus, since the extent of warping increases with the dimension of the panel board, while only a thin layer of the tread surface may be removed by finish-machining, the length of the panel board is very limited for this reason.
Furthermore, it is obvious that the warping which occurs interferes with precise milling of the slots in the crossband ply.
The panel board manufactured by this known process has the advantage of being relatively thin, so that when it is laid on a cast plaster floor it conforms to any minor unevennesses that may be present and does not, as in the case of a thicker panel board, span over these in unsupported manner, so that the known panel board does not flex during local load.
In order to simplify the expensive manufacture of multi-ply panel boards, a process is also known in which, as in the aforesaid, known process, the tread ply is glued with a crossband ply of wooden boards or slats, the fiber grain of which extends at right angles to that of the tread ply.
On the surface of the crossband ply facing away from the tread ply, there is now glued a further ply, the fiber grain of which matches that of the tread ply and which is intended to absorb tensile stresses, whereby the above-described "dishing" as well as the above-described warping that occurs during gluing is intended to be counteracted.
However, in order to prevent the situation, for the aforesaid reasons, that the panel board becomes too thick, there is used in practice, as the further ply, a rotary-cut veneer, the cross section of which is much too small in order actually to absorb such large forces as are necessary to counteract warping effectively.
Thus expensive finish-machining by material-removing processes is also necessary for this known panel board, if warping has occurred during gluing.